Increased demands on the running qualities of gears in gear drives require more and more the precision finish machining of the gear teeth. In most cases the pre-machined workpieces are case-hardened, and then fine machined to the finished dimensions on all their functional surfaces. In this process the fine machining of the tooth flanks in particular is a complicated, resource consuming, and hence expensive operation. In the interests of economic manufacture, but also in order to avoid having to provide for unnecessarily large hardening depths and to balance and minimize the wear on the left and right flanks of the fine machining tool, it is therefore attempted to keep the material allowance for the fine machining as small as possible. For the fine machining of the teeth this means in practice that the depth of cut per flank for material removal is only a few hundredths to at most two tenths of a millimetre. If, as is generally the case, the left and right flanks are machined in the same operation, this demands a very accurate alignment of the precut teeth relative to the fine machining tool, so that the latter can be brought exactly into the centre of the tooth spaces to be machined, in order to achieve uniform material removal from the left and right flanks of the workpiece.
In the aligning process most frequently encountered in practice on gear finishing machines, a non-contact functioning measuring probe, the aligning probe, is located at a geometrically suitable point in the working area of the machine. It is preferably positioned near the outer diameter of the pre-cut teeth of the workpiece set up on the work spindle. This aligning probe usually operates on an inductive, optical or magnetic principle. For the alignment the work spindle is rotated, and the angular positions of all the tooth flanks of the gear blank to be machined are measured by the aligning probe. Subsequently the average value of all the tooth space centre lines is calculated. This average value is compared with a desired value stored in the machine control system. The workpiece can now be aligned by turning the work spindle through an angle corresponding to the difference between the measured average value and the pre-defined desired value; that is, the average value and the desired value are brought to coincide in the machine control system.
The desired value of the angular position of the tooth space centre relative to the machining tool is determined by way of a so-called teach-in cycle during the setting up of the machine. To this purpose, an alignment measurement is performed on a workpiece which has been machined shortly beforehand, and which is still on the work spindle. Provided that both flanks of this workpiece are cleanly machined at the measuring point, i.e. both flanks are completely machined, in particular ground, these flanks represent an exact desired position of the tooth space centre relative to the machining tool. The desired value is then that angular position that is measured in this alignment measurement.
One of the pre-requisites for the attainment of a high aligning accuracy with the described procedure is that the measuring conditions for the aligning procedure during series production do not change relative to those during the machine setting process, or during the determination of the desired alignment value in the teach-in cycle. Practical experience shows, however, that in the course of series production aligning errors occur according to a time trend due to thermal or other physical influence. The result is differing material removal on the left and right flanks, or scrap as soon as due to the scant machining allowance the finish machining operation fails to embrace the entire flank surface on some flanks.
On account of the complex machine construction and processing sequence, and of the numerous physical factors of influence intrinsic to the process, the causes of such alignment errors varying to a trend pattern during series production can mostly only be determined and brought under control at great expense. In some cases the only method is the interruption of the production process and the repetition of the teach-in operation by the setter or the operator, by which production is considerably impaired and made more expensive. The necessity for this is often not realized until a substantial amount of scrap has already been produced.